Loaded submarine cable



Jan. 5, 1932. J. .1. GILBERT 1,840,089

LOADED SUBMARINE CABLE Filad Aug. 20, 1927 lhllll Patented Jan. 5, 1932UNITED-STATES PATENT orifice JOHN J. GILBERT, 0F DOUGLASTON., NEW YORK,ASSIGNOR TO WESTERN ELECTRlC' kCOM- .'PANY, INCORPORATEDjOF NEW YORK, N.Y.,` A CORPORATION 0F NEW YORK LOADED' SUBMARINE CABLE Application filedAugust 20, 1927. Serial No'. 214,425; i

This invention relates to long submarine cables of the deep sea typeandmore particularly tocoil loaded cables designed for direct current orMorse operation. For the purpose of this specification theenpressiondirect current transmission systems will be used todistinguish such systems .from carrier wave transmission and telephonesystems.

Among the objects of the invention are, to improve direct Currenttransmission over long submarine cables, to reduce disturbances at thereceiver, to reduceduplex imbalance,

to reduce dielectric losses,f'and to more e flec-k tively utilizemagnetic loading materials. Coil or lumped loading of aeriallines7especially carrier telegraphlines and telephone conductors has beenpracticed to a considerable extent. .No parallel development of coilloaded direct currenttelegraph cables of the deep sea type has takenplace. i Numerous diculties have been recognized as preventing suchdevelopment, one of which is the diiiiculty. of manufacturing andlaying` on the seat bottom a coil loaded cable. Each coil not onlyinvolves a large additional expense but vintroduces additionalprobability of breakage or of insulationfailure and thereby imperils the'whole projectj `For this reason it is extremely desirable thatthenumber of coils employed in a coil loaded direct current submarinecable be such as will enable transmission and reception to be carried.on-satis 'factorily but that nogreater number be used.

It has long been-known thatone can simulate va continuously loadedconductor with any desired degree of approximation by spacingproperlydesigned loading coils sufli-r ciently close together. Rules kforloading telephone and carrier current conductors have been 'formulatedand appliedein'sensive` ly with entire success. By experiment, it hasbeen determined that lthe usualrules for loading telephone andjcarrierconductors do not fulfill the conditionsV of effective highl speeddirect current telegraph transmissionover long submarine cables.; Inaccor: ance with modern practice, ampliers of great gain are employedfor amplifying the impulses re-Y ceived over such cables-and-undersuchconditions the usual' loading practices result in troublesomephenomena at the4 receiver. `These phenomena yare duepartlyfto'reflection from the coils in the vicinityr of the receivingend and partly to an accumulation of reliections as thesignal istransmitted over the cable. Such reflections have been styled filteroscillations.y In duplex transmission similar phenomena occur inconnection with the operation of the local receiver. However, this is asecondary considerationand the in- `ventionrelates primarily toreduction of the effects offilter oscillations at the receiver.

As stated above, it is known that a lump loaded line may be madetosimulate a smooth loaded line within any desired degree ofapproximation. In telephone practice, this rule is carried into edect byemploying 8 ci' 9, or in some cases7 slightly more or slightly less,

,coils per wave length atthe highest frequency to be transmitted.Althoughy the use citl various extremely large numbers of coils per wavelength has been theoretically discussed .for the` purpose of showingmathematicallyr the degreev of exactitude to which a smooth yline ma bea roximated .such reater Y 7 number has been regarded as simulating asmooth, line with unnecessarily great precision. The usual practices forloading telephone and high frequency carrier lines are not applicabletodirect current telegraph transmission over long deepsea cables becausethe phenomena of filter oscillations introduce new considerations.

For directcurrent telegraph operation it is 4usual to'deline thesignaling frequency as lthe fundamental frequency of a series ofalternate positive and negative impulses of unit 4 length.

This definition is used here. It is usually considered, in modernsubmarine cable telegraph practice, that it isnecessary to l transmitwith attenuation not excessively greater than the signaling frequencyall frequencies up to 11/2 times the signaling frequency. Frequencieshigher than this may be and usually are considerably attenuated. Thisassumption is a rather approximate one, however, because the attenuationcurve of an actual cable rises steadily and at an increasing rate inpassing to higher frequencies from the signaling frequency. It ispossible to calculate or measure the attenuation for any given frequencybut difficult to say at what frequency the attenuation becomes sok greatas to render the energy of that frequency negligible. Assuming ll/gtimes the signaling frequency to be the highest frequency to betransmitted. it would be proper, in accordance with telephone practice,to load the cable so that the theoretical cut-off frequency would beabout 2 to 21/2 times the signaling frequency, the particular valuedepending upon the quality of the telephone loading system chosen forcomparison. This would necessitate about 7 to l() coils per wave length.However, such coil spacing for long submarine cables employed for directcurrent telegraph transmission leads to diliiculties on account of thefilter oscillations hereinbefore mentioned. Such filter oscillations`corresponding to the most fundamental form of l transmitted signal, thatis. an abrupt change in D. C. voltage applied to the cable, consist of atrain of oscilla-tions supcrposed on the usual arrival curve, the periodand amplitude of the successive elements of the train being ofsuccessively decreasing magnitude. The first 1.@ cycle, or moreaccurately l/ wave, of such oscillations produces the principaldifficulty at the receiver. A large component of the energy of this -l/Qwave is concentrated in a frequency region corresponding roughly to 1,4,the theoretical cut-off frequency. lilith a coil spacing of 7 to l0coils per wave length, a large part of the energy of these oscillationsfalls within a. range close to the signaling frequency. The troubledueto this source is aggravated by the modern tendency to the use of high gain amplification. However, b employing from 15 to 20 coils per wavelength7 the cut-off frequency may be caused to fall between 5 and Gtimes the signal frequency and the energy of the troublesome l@ wave ofthe filter oscillation will consist largely of energy of frequency justabove 11/3 times the signaling frequency and may be suppressed byselective circuits or other means. In practice the attenuating propertyvof the cable for such energy and the use of selective circuits at thereceiver to discriminate against it both have a beneficial effect. Thenumber of coils per Wave length (because of the economic considerationsdiscussed above) should be the least possible consistent with reducingto a permissible value the amplitude of the first 1/2 wave of the filteroscillations.

This rule of 15 to 20 coils per wave length applies to the case whereunit impulses are transmitted and received. In accordance with ak moremodern type of system, the speed of transmission is effectively doubledby setting the signaling frequency at such a value that impulses oftwice unit length are the least which are transmitted over the cable andactuate the receiving relay or other receiving device. Single impulsesof effective amplitude are not received but are produced at theJreceiver by regeneration. In such a system the signaling frequency forthe purposes of the present invention and in so far as the receiver isconcerned will be that at which alternations of impulses of twice unitlength are transmitted. The signaling frequency corresponding to theunit impulses is disregarded and the frequency' of transmission asestablished by the shortest impulses which are actually received at thereceiver is regarded as the signaling frequency for the purposes of thisinvention.

An advantage of coil loaded cables is increased ease in constructing anartificial line. In the present instance the artificial line willpreferably have the number of coils per wave length exactly the same asin the cable conductor.

In general, with coil loaded cables, duplex balance is facilitated bythe ease with which an artificial line may be constructed to balance thecable. Reflections from cable loading coils may be neutralized bycorresponding reflections from coils of the artificial line which willbe of the same amplitude and can by careful design be made to have thesame phase, within adesired degree of approximation.

Inasxnuch as the filter oscillation phenomena at the receiver are duepartly to reflections from coils in the vicinity of the receiving endand partly to an accumulation of reflections as the signal istransmitted over the cable, it would be permissible to use greater coilseparation at the mid-portion of the cable than near the terminals.However, if greater spacing is used at the mid-portion the 'spacing inthe terminal sections should be reduced to compensate for it.

The word coil is used in the preceding discussion in the usual sense. Itmay be desirable, however, to provide an inductance lump consisting ofa. straight conductor a few hundred or a few thousand feet in lengthcontinuously loaded with several layers of loading material in the formof wire or tape helieally applied. These inductance lumps should be of atotal diameter over the copper conductor and loading equal to thediameter of the non-loaded core. Of the total length a portion less than1/2 and preferably about 1/10 should consist of continuously loadedportions. These coils or inductance lumps should be uniformly spaced adistance vof 1/20 Orl/15 wave length.y krVlith fsuch an arrangement thecable structure willV described in ElmenPatent, 1,715,647, granted J une4, 1.928) which have constantpermeability Yfor, a magnetizing force upto several c. g. s. units. They comprise compositions of iron, nickeland cobalt, with or without other ingredients,heat treated to developthe desired constancy of permeability. By using elongated coils loadedwith several layers of such magnetic materials, the portion of themagnetization curve which is straight may be much more effectivelyutilized than in the case of conductors continuously loaded throughout.This is done by making the flux density greater inthe loading materialthan in the case of continuous loading Y throughout.

lt is desirable to employ insulating materials of leakance lower thanthat of gutta percha immediately adjacent the conductor of a high speec`submarine cable. er part of the energy losses in the dielectric takeplace in the region immediately adjacent the conductor and in the caseof the lump loaded structure just described where a large portion, forexample 9/10 of the conductor is non-loaded, advantage Ycan be taken ofthis fact to apply a layer of such low leakance material immediatelyadjacent the unloaded portion of the conductor. ln the case ofcontinuously loaded conductors, the application of such a layer` ofmaterial is more difficult on account of the necessity of comlpletelysurrounding the loading material by a semi-fluid pressure equalizingmedium. Of course, the material of the lowestleal-:ance which it isfeasible to apply should be applied to the loaded portion of theconductor to serve as a pressure equalizing medium for the loadingmaterial.

Artiiicial lines for balancing conductors loaded with from to 20`coilsper wave length may be conveniently constructed with a. correspondingnumber of coils per wave length. To the extent that filter oscillationsset up in artificial lines are troublesome, the use of the invention maybe advantageous in causing their reduction.

Since accuracy of duplex balance is largely dependent upon balancing afew hundred miles of the cable near the terminals, the form ofconstruction employing coil loaded end sections and a continuouslyloaded interv mediate portion offersl considerable advan- The greatytages. In this case, actual coils instead of iii ;luctance lumps, formedby continuously loaded sections, are preferred. The design is such thatthe impedances at the Junction of the two kinds of cable are matched asaccug rately as possible over the range of frequencies to betransmitted.

' The mode of applying the invention is described in further detail byreference to the accompanying drawings in which Fig. 1 represents asystem iii accordance with the in-v vention designed for transmitting inone direction at a time;

Fig.2 represents a modified forni of the invention adapted forduplexoperation; and

Fig.y 3 represents a cable loaded with loading coils near the terminalsand continuously in the middle portion.

ln Fig. l a. switch 10 serves to transfer manually or automatically thecable terminal 5,

back and `forth between a transmitter and a receiver and the switch 10performs the same function at the distant terminal. The trans- 'niittersT and T1 are any ordinary type and the receivers R and R1 are equippedwith:

curacy by known methods.y In the case of 2000 mile cable havingmilli-lienrys inductaiice and .36 microfarads inductance per nauticalmile the coil spacing at 8() cycles signalingv frequency should not begreater than four nautical miles, but preferably` slightly less thanthis.

nig. 2 isarranged for duplex operation. The transmitter T comprises animpulse sender S which is connected to a transmitter relay TR by meansof a filter network F. The relay TR is actuated by windings 13 and 1li.Filter F suppresses impulses of unit length for which reason the relayTR is actuated only by impulses of twice unit length and impulses ofunit length are not impressed upon the cable conductor. The receivercomprises a vacuum tube amplifier with suitable correcting networksincorporated therein having its output circuit connected-to aregenerative i'elay RR. The regenerative relay operates in a well knownfashion to interpolate or restore theimpulses of unit length suppressedby the filter F remployed in connection with the transmitter. The cablecomprises long unloaded sections 15 and short continuouslyloadedsections 16. The loaded sections are spaced apart about 1/20 to 1/15wave length at thefundamental signaling frequency impressed upon theconductor by' the relay TR.y The loaded sections function 4asinduct-ance lumps or loading coils and are loaded with several layers ofmagnetic material 17 applied helically in the form of Wire or tape. Thecopper core of the loaded portions is reduced in diameter and theloading material is of such thickness that the total diameter of theloading material is just equal to the diameter of the loaded portions.The loading material may be a nickel-iron alloy for example, orpreferably a magnetic alloy of iron, nickel and cobalt `with or withoutother ingredients and heat treated to have a constant permeability up toa magnetizing force of 2 or 3 gauss, more or less. rlie loading materialis thoroughly impregnated with bitumen 18 or other suitable pressureequalizing substance. On the unloaded copper core may be applied a thinlayer 19 of vulcanized or depolymerized rubber or other material havinga leakance lower than that of gutta percha. The entire core includingthe loaded and unloaded portions may be surrounded by a layer 20 ofgutta percha or other suitable insulation. The entire cable issurrounded by jute and armored in the usual fashion.

In Fig. 3 is illustrated a cable with end sections leaded with coils 21,21 and an intermediate continufnisly loaded section. The coils may be ofany suitable type but for duplex Working' are preferably relativelyshort coils as distinguished from elongated inductance lumps. Forone-way operation the coils may be short coils or loaded sections suchas 16, 1G of 2. The cable is designed so that the in'ipedances of thecoil or lumped and continuouf-ily loadA :l portions are matched withintia` band of e "ential frequencies to be transmitted. The loading coilsare spaced 15 to 20 per wave length and the coils of a balancingartiiicial line are similarly spaced in the hea d end portion ot theline but further ont may have some other suitable spacing if desired.

The details of the invention and modes of its application in practicehave been described. The novel features inherent therein are set forthin the appended claims.

What is claimed is:

1. A. submarine cable in which a portion near the ends magneticallyloaded by means of loading;` coils and a` portion near the centralpartis magnetically loaded by magnetic material Wrapped about extendedand relatively long portions of the central conductive core.

2. A submarine cable section comprising a central conductive core havingrelatively small portions of the core continuously loaded with magneticmaterial, the loaded portions being uniformly spaced apartandconstituting less than one-half the total length of the cable section inwhich the diameter of the loaded portion ef the conductive core isreduced as compared to that of the unloaded portion, whereby the totaldiameter of the core plus the loading material of the loaded portionsapproximates that of the unloaded portions.

A submarine cable signaling system in which impulses of unit lengthproduced at the transmitter are too attenuated at the receiver to beeffective and are reproduced by regeneration, in which the cable isloaded with from 15 to 20 coils per wave length at the signalingfrequency determined by the transmission of alternate positive andnega.- tive impulses of twice unit length.

In witness whereof, I hereunto subscribe n y name this 17th day ofAugust, A. D. 1927.

JOHN J. GILBERT.

